1. Field of the Invention
The present invention relates to an optical film, a polarizing plate using the optical film, and an image display apparatus using the optical film or the polarizing plate.
2. Description of the Related Art
An optical film, particularly, an antireflection film is generally positioned on the outermost surface of a display in order to prevent contrast reduction due to reflection of external light or reflection of images in an image display device such as a cathode ray tube display device (CRT), a plasma display (PDP), an electroluminescence display (ELD), or a liquid crystal display apparatus (LCD). For this reason, the antireflection film is required to have, other than high antireflection performance, high transmittance, high physical strengths (such as scratch resistance), chemical resistance, weather resistances (such as wet heat resistance and light resistance).
Such an antireflection film often employs the following: a low refractive index layer which is a thin film layer with a layer thickness of 200 nm or less is provided at least on the outermost surface, so that antireflection is effected by optical interference of the low refractive index layer. However, for one layer thin film interference type in which antireflection is effected with one layer of the low refractive index layer of the simplest structure, there is no practical low refractive index material satisfying a reflectance of 0.5% or less, and having a neutral tint, high scratch resistance, chemical resistance, and weather resistances. In contrast, there is known a multilayer thin film interference type antireflection film for preventing reflection by multilayer optical interference, such as a two layer thin film interference type in which a high refractive index layer is formed between a transparent support and a low refractive index layer, or a three layer thin film interference type in which an intermediate refractive index layer and a high refractive index layer are successively formed between a transparent support and a low refractive index layer. Particularly, for preventing reflection in a wide wavelength range, and implementing low reflectance while suppressing reflected color, the three layer thin film interference type is desirable.
As the thin film layers (such as a high refractive index layer, an intermediate refractive index layer, and a low refractive index layer) for use in the multilayer thin film interference type antireflection film, multilayer films including transparent thin film layers of metal oxides in a lamination have been widely used in the related art. In general, the transparent thin films of metal oxides are formed by a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method, particularly, a vacuum deposition method which is one kind of the physical vapor deposition methods, or a sputtering method.
However, the method for forming transparent thin films of metal oxides by vapor deposition or sputtering is low in productivity, and is not suitable for mass production. Thus, there is proposed a method of formation by a wet deposition method high in productivity, particularly a coating method.
Alternatively, there is also commonly employed the following: an antiglare layer having an uneven shape is formed in order to prevent the reduction of the visibility due to reflection of external light, or reflection of a fluorescent lamp or the like. The formation of the antiglare layer having an uneven shape is generally accomplished in the following manner. On the surface of a transparent base material, a resin containing inorganic oxide fine particles of silica or the like, or organic resin beads is coated for formation. However, while the formation of the antiglare layer can prevent reflection with ease, the following phenomenon occurs: the screen looks whitish due to surface scattering of external light, and tightness of black is degraded. Under such circumstances, the following is carried out: one layer of a thin film interference layer is provided on the antiglare layer, thereby to implement both prevention of reflection and tightness of black.
In order to further improve the reflection and the tightness of black, it is preferable to form two or three thin film interference type antireflection layers on the antiglare layer. However, when a thin film on a nanometer scale is tried to be coated on the uneven surface of the antiglare layer by the foregoing coating method good in productivity, nonuniform coating or cissing occurs due to the unevenness. In some cases, this results in nonuniform film thickness, so that optimal light interference does not occur as designed, and thereby the reflectively is not reduced. For this reason, it is difficult to implement both the low reflection and the antiglare property.
In order to improve such nonuniform coating and cissing, the following means are tried: after surface treating the antiglare layer surface, an aging treatment is carried out (JP-A-2006-145737); and an intermediate layer is provided between an antiglare layer and an antireflection layer (JP-A-2006-145587). Alternatively, the following means is tried: hollow spherical silica fine particles in a low refractive index layer are arranged so as to be present in a larger amount at the concave parts than at the convex parts (JP-A-2006-146027).
However, the optical films proposed in the related art are not enough for reducing the reflectance.